This invention relates to an information recording disk comprising a disk-like substrate and recording layers formed on both surfaces of the substrate.
Recently, a vertical magnetization recording system utilizing a residual magnetism in a direction perpendicular to the main surface of a magnetic recording medium attracts attentions in this field. In this system, the magnetism is unlikely to be attenuated by the adjacent magnetic field, making it possible to perform a high density recording. A typical magnetic recording medium for the perpendicular magnetic recording, hereinafter referred as the perpendicular magnetic recording medium, consists of a substrate and a Co-Cr series thin film formed on the substrate. The thin film has a magnetizable axis in its thickness direction and acts as a magnetic recording layer. Also, the substrate is formed of a polymer film such as a heat-resistant film consisting of polyimide or a polyethyleneterephthalate film. The perpendicular magnetic recording medium is prepared by forming the thin films on both surfaces of the substrate by means of sputtering, and used in general as a floppy disk. When used as a floppy disk, the substrate is of a disk shape.
The Co-Cr series thin films are formed on both surfaces of the substrate as mentioned above. If the thin film is formed on only one surface by sputtering, the substrate is markedly curled such that the central portion of the substrate projects toward the film-formed side. If the thin films are formed on both surfaces, however, the curling generated in the step of forming a first thin film is substantially corrected in the step of forming a second thin film, with the result that the produced recording medium is substantially free from the curling.
The curling may be completely eliminated theoretically, if the thin films are formed under the same sputtering conditions and have the same thickness. However, it is practically impossible to completely eliminate the curling, even if the Co-Cr series thin films are formed on both surfaces under the same sputtering conditions and in the same thickness. This is because the substrate in the second sputtering step, i.e., the substrate having a first thin film formed thereon, differs from the single substrate in properties such as the effective thermal expansion coefficient and Young's modulus. To overcome the problem, it may be necessary to make the first and second thin films different from each other in thermal expansion coefficient and mechanical properties by changing the thickness, sputtering conditions and composition of the first and second magnetic thin films.
However, if the curling is corrected in this fashion, the first and second magnetic layers are made different from each other in magnetic properties such as a recording density and coercive force, making it necessary to distinguish the first and second layers by some means. When it comes to a thin film type recording medium of a certain kind or to a coating type magnetic recording medium in which magnetic layers are formed on both surfaces of the substrate under the same conditions and, thus, have exactly the same properties, the first and second magnetic layers need not be distinguished. However, where the first and second magnetic layers formed on both surfaces of the substrate differ from each other in magnetic properties, it is necessary to use only the magnetic layer having satisfactory properties as the recording layer. Alternatively, the recording-reproduction must be performed under the conditions adjusted to meet the magnetic properties of the magnetic layer. It follows that it is necessary to distinguish the front and back sides of the recording medium and to load the recording medium in a package, with the recording medium and the package aligned with respect to the directivity.
However, the front and back surfaces of the magnetic recording medium produced as above seemingly exhibit the same metal luster, making it impossible to visually distinguish the front and back surfaces. Thus, it was customary to apply total inspection to the recording medium before loading in a package in order to avoid confusion in the loading step with respect to the front and back sides of the recording medium. What should be noted is that the total inspection requires much time and labor in the manufacturing process of the magnetic recording medium, leading to a low productivity of the recording medium.